There is nothing to stop you and your friend having any agreement you like. It would be better done by deed which needs to be witnessed because that makes it more official.
Even if the friend will not agree and whoever has the property at the moment and who you are looking after decides to cut you out of the will in favour of your friend, and you spent money and time looking after the personal, and you did so on the basis that they promised they would leave you the property, they would be denied honouring the promise under a legal doctrine called Promissory Estoppel.
You need to have had a promise made to you and then relied on that promise to your detriment which, in this case, you would have done by spending time and money.
So you see, there are actually 2 ways of dealing with this.
It would be essential that your friend takes legal advice from an independent lawyer on the effect of entering into the agreement for it to be enforceable.
Can I clarify anything for you?
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I can still clarify things for you.
I can provide you with a deed but there is an extra cost for it. I will submit a premium services proposal. A solicitor would probably charge you about £250 for doing the document but I would be able to do it for a fraction of that.
You need the parties names.
The recital which is basically the background and what the deed is giving effect to
The body of the agreement and confirmation that each party is taking legal advice
And then the execution clauses along with witness details